Uninterruptible power system

ABSTRACT

A boost circuit is connected to a rechargeable battery and a switch is connected between a rectification circuit and a power factor correction converter. In the power failure state, the switch is turned on to activate the boost circuit and energy of the rechargeable battery is supplied to a load by rasing the voltage at two steps in the path from the boost circuit, switch, and to power factor correction converter. Since the voltage of the battery can be lowered, the number of serially connected cells can be made small. The volume of UPS can be reduced and a low cost can be realized. Since the number of serially connected cells is reduced, the reliability against failure of battery cells can be improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is relevant to U.S. patent application Ser. No.10/412,231 being filed by Akihiko Kanouda, Minehiro Nemoto, FumikazuTakahashi, Masahiro Hamaogi, Yoshihide Takahashi, Takashi Tanabe, TakaoGotou, Masato Isogai, Toshikatsu Miyata, and assigned to the presentassignees, based on Japanese Patent Application No. 2002-113116 filed onApr. 16, 2002, and U.S. patent application Ser. No. 10/412,319 beingfiled by Minehiro Nemoto, Akihiko Kanouda, Fumikazu Takahashi, MasahiroHamaogi, Yashihide Takahashi, Takashi Tanabe, Takao Gotou, MasatoIsogai, Toshikatsu Miyata, and assigned to the present assignees, basedon Japanese Patent Application No. 2002-113117 filed on Apr. 16, 2002.The contents of the applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(A) Field of the Invention

The present invention relates to an uninterruptible power system (UPS)capable of supplying a stable power to a load even when an AC powerfailure occurs.

(B) Description of Related Art

When a power failure occurs, damages such as data loss are feared in aso-called information processing apparatus such as a server, a routerand a storage. As countermeasures for power failure, an uninterruptiblepower system (UPS) has been installed which uses an energy accumulationunit such as a rechargeable battery. UPS includes an AC output type anda DC output type. The former is more popular and inserts anuninterruptible power system between a commercial AC power source and asubject apparatus. Basing upon the DC output characteristics, the lattergenerally adopts the structure that an uninterruptible power system isconnected to a DC line at the succeeding stage of an AC/DC converter ofan apparatus. Examples of the latter are disclosed in Patent Document 1and Japanese Patent Laid-open Publication No. 2002-171692.

According to these conventional technologies, an AC power is suppliedfrom an AC power source to a rectification circuit and to a power factorcorrection circuit. A DC power is then supplied to a DC output DC/DCconverter whereat the DC power is converted into a plurality of DCpowers having desired voltages and currents which are supplied to loads.In a power failure state, a DC power is supplied from a battery to abackup converter and to the DC output DC/DC converter. In a power normalstate, the battery is charged with a charger.

Patent Document 1:

JP-A-2000-116029

(Abstract and Others)

In a conventional backup power source, during a power failure, a DCvoltage, e.g., 380 V, is required to be generated from a battery via abackup converter. It is conceivable that a lead battery is often used asthe backup battery. The voltage of a lead battery per cell is about 12V. From recent requirements for compact and light weight batteries, itis supposed that rechargeable batteries having a higher energy densityare used, including nickel metal hydride (NiMH) batteries, lithium-ion(Li-ion) batteries and the like. The NiMH battery is about 1.2 V percell, and the Li-ion battery is about 3.7 V per cell. It is alsoexpected to use a high capacity charge accumulation unit such as anelectric double layer capacitor and a fuel battery expected to be inpractical use in the future.

As a backup converter, a non-isolated, boost chopper type converter maybe used which has a relatively simple and compact structure and is easyto be controlled. A ratio of a low side voltage to a high side voltageof this converter is required to be suppressed to a range from about 1.2times to about 3 times when the circuit efficiency is taken intoconsideration. The number of battery cells is therefore about 105 to 264cells for an NiMH battery and about 34 to 86 cells for a Li-ion battery.

Such a large number of serially connected battery cells poses someproblems of a variation in States of Charges (SOC) of cells and thereliability against occurrence of a cell shortage failure. From thisreason, system sides require that the number of serially connected cellsis to be suppressed to 30 cells or less at the most.

In order to reduce the number of serially connected cells, it isessential to realize a high boost ratio of an isolated converter using atransformer. However, this results in not only a complicated structurebut also a high cost and a large size.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an uninterruptiblepower system capable of setting relatively low a voltage of an energyaccumulation unit such as a battery.

It is another object of the present invention to provide anuninterruptible power system capable of configuring an energyaccumulation unit with the appropriate number of serially connectedcells of about 30 cells or less, even if a high energy densityrechargeable battery, a high capacity charge accumulation unit or a fuelbattery is used.

It is yet another object of the present invention to provide anuninterruptible power system capable of using a non-isolated, boost typeconverter having a relatively simple and compact structure and beingeasy to control.

It is yet another object of the present invention to provide anuninterruptible power system capable of using a battery charge circuitwith a simple structure or not using the battery charge circuit.

According to one aspect of the invention, a power in energy accumulationmeans is passed through serially connected two boost means to beconverted into a desired power and supplied to a load.

According to another aspect of the invention, an uninterruptible powersystem comprises: a power factor correction boost converter forreceiving an output of a rectification circuit, suppressing inputcurrent harmonics by switching control, and supplying a power to anoutput converter; and boost means for boosting an output of energyaccumulation means to feed a power to the output converter, wherein apower in the energy accumulation means is supplied to an input side ofthe boost converter via boost means.

A relatively low voltage of energy accumulation means can be boosted toa desired voltage by two-stage boost means. If the power factorcorrection boost converter is used, already present boost means can beefficiently utilized so that the system can be made simple, compact andinexpensive.

According to another aspect of the invention, a portion of an outputside of the output DC/DC converter is coupled to a high voltage side ofthe boost means, the boost means is a bi-directional DC/DC convertercapable of a voltage lowering operation in a reverse direction, theboost means charges the energy accumulation means while the AC powersource is normal, and while the AC power source fails, the boost meansperforms a voltage raising operation to control the discharge from theenergy accumulation means to the input side of the boost converter.

The bi-directional DC/DC converter can be effectively used for thecharge/discharge control both in the power failure state and powernormal state. The system can be further made simple, compact andinexpensive.

According to another aspect of the invention, a voltage on a highvoltage side of the bi-directional DC/DC converter is set higher whilethe AC power source fails and is backed up than while the AC powersource is normal.

Accordingly, the bi-directional DC/DC converter can be used also for thecharge operation in the range determined by a voltage raising factornecessary for the AC power failure, without making the system bulky.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of an uninterruptiblepower system according to an embodiment of the invention.

FIG. 2 is a block diagram showing the structure of an AC power failuredetection circuit according to the embodiment of the invention.

FIG. 3 is a timing chart showing voltage and operation waveforms in eachcircuit portion according to the embodiment of the invention.

FIGS. 4A and 4B are diagrams illustrating the flows of electric energyaccording to the embodiment of the invention.

FIG. 5 is a circuit diagram showing the specific structure of theuninterruptible power system according to the embodiment of theinvention.

FIG. 6 is a diagram showing the structure of an uninterruptible powersystem using a fuel battery according to a second embodiment of theinvention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a block diagram showing the structure of an uninterruptiblepower system according to an embodiment of the invention. A commercialAC power source 1 feeds a power to a load 3 via an uninterruptible powersystem (UPS) 2. In the uninterruptible power system (UPS) 2, a noisefiler 4 is provided connected to the AC power source 1. An output of thenoise filter 4 is supplied to a full wave rectification circuit 5. Anoutput point (a) of the full wave rectification circuit 5 is connectedto a power factor correction converter 6. This power factor correctionconverter 6 is a boost type converter for suppressing input currentharmonics through switching control, the details of this converter beinglater described. DC 380 V appears at an output point (b) of the powercorrection converter 6. DC 380 V is applied to a multi-output DC/DCconverter 7 which feeds powers having different voltages to the load 3.In this example, 5 V, 12 V and 3.3 V are applied to the load. Themulti-output DC/DC converter 7 has another output point (c) connected toa diode 77 to be described later, the output point being connected to acharge/discharge circuit 8. In a normal state of the AC power source 1,the charge/discharge circuit 8 lowers the voltage at the output point(c) of the multi-output DC/DC converter 7 to charge a rechargeablebattery 9.

A serial connection of a switch 10 and a diode 11 is connected betweenthe output point (c) of the charge/discharge circuit 8 and the inputpoint (a) of the power factor correction converter 6.

An AC power failure detection circuit 12 is connected to the input pointof the power factor correction converter 6, i.e., the output point (a)of the full wave rectification circuit 5. An output of the AC powerfailure detection circuit 12 controls to turn on and off the switch 10,and supplies a control instruction to a charge/discharge control circuit13.

The operation of the embodiment will be briefly described. In the normalstate of the AC power source 1, an AC power is supplied via the noisefilter 4 to the full wave rectification circuit 5 to be rectified andthen supplied to the power factor correction converter 6. The powerfactor correction converter 6 flows current proportional to theamplitude of an output voltage of the full wave rectification circuit 5to boost the voltage, for example, to DC 380 V. An output voltage 380 Vof the power factor correction converter 6 is applied to themulti-output DC/DC converter 7 so that DC voltages of 5 V, 12 V and 3.3V are applied to the load 3.

DC voltage of about 48 V to 54 V is applied to the charge/dischargecircuit 8 from the output point (c) of the diode 77 in the multi-outputDC/DC converter 7. The AC power failure detection circuit 12 maintainsthe switch 10 open because a power failure is not detected presently,and it instructs the charge/discharge control circuit 13 to charge therechargeable battery 9 by using the charge/discharge circuit 8.

FIG. 2 is a block diagram showing an example of the structure of the ACpower failure detection circuit 12 in UPS 2 shown in FIG. 1, accordingto the embodiment of the invention. The output point (a) of the fullwave rectification circuit 5 shown in FIG. 1 is connected to aninverting input terminal of a hysteresis comparator 120 in the AC powerfailure detection circuit 12. A reference voltage source 121 isconnected to a non-inverting input terminal of the hysteresis comparator120. An output of the hysteresis comparator 120 is connected to a filter122 and a clear terminal of a counter 123. An output of the filter 122is applied to a reset terminal of an RS flip flop circuit 124. A clockgenerator 125 is connected to a clock terminal of the counter 123, andan output of the counter 123 is supplied to a D/A converter 126. Anoutput of the D/A converter 126 is applied to a non-inverting inputterminal of a comparator 127. A reference voltage source 128 isconnected to an inverting input terminal of the comparator 127. Anoutput of the comparator 127 is applied to a set terminal of the RS flipflop circuit 124. An output Q of the RS flip flop circuit 124 is outputto an external of the AC power failure detection circuit 12 and appliedto the charge/discharge control circuit 13 and to the switch 10.

Next, by referring to FIG. 3 and FIGS. 4A and 4B, description will begiven on the operation of the embodiment described with reference toFIGS. 1 and 2.

While the commercial AC power source 1 is in the normal state, a poweris supplied from the AC power source 1 to the full wave rectificationcircuit 5 via the noise filter 4 and rectified at the full waverectification circuit 5.

FIG. 3 is a timing chart showing voltage and operation waveforms in eachcircuit portion according to the embodiment. A voltage at the outputpoint (a) of the full wave rectification circuit 5 is input to the powerfactor correction converter 6. The power factor correction converter 6flows current proportional to the amplitude of an output voltage at theoutput point (a) of the full wave rectification circuit 5 to boost thevoltage. An output waveform at the output point (b) has, for example, aconstant DC 380 V as shown in FIG. 3. The output voltage 380 V isapplied to the multi-output DC/DC converter 7 so that DC voltages of 5V, 12 V and 3.3 V are applied to the load 3. DC voltage of about 48 V to54 V is applied to the charge/discharge circuit 8 from the output point(c) of the multi-output DC/DC converter 7.

At this time, the voltage at the output point (a) of the full waverectification circuit is applied to the hysteresis comparator 120 in theAC power failure detection circuit 12. In the actual case, a dividedvoltage of the voltage at the output point (a) is applied to thehysteresis comparator, since the breakdown voltage of the hysteresiscomparator 120 is about several V to 15 V. The voltage at the outputpoint (a) is illustratively shown in FIG. 2. A voltage of the referencevoltage source 121 to be applied to the non-inverting input terminal ofthe hysteresis comparator 120 is lower than the peak value of the outputat the output point (a) of the full wave rectification circuit. As shownin FIG. 3, an output of the hysteresis comparator 120 has therefore apulse waveform having a frequency as twice high as the AC power sourcefrequency. Clocks of the clock generator 125 having a frequencysufficiently higher than the AC power source frequency are supplied tothe clock input terminal CLK of the counter 123 to count up the counter123. An output of the counter is reset when a pulse waveform output fromthe hysteresis comparator 120 is input to the clear terminal CLR of thecounter 123. An output of the counter 123 is converted into an analogvoltage by the D/A converter 126. An output of the D/A converter 126 hasthe waveform indicated by “126 OUTPUT” in FIG. 3. The counter 123 is sodesigned that it is reset before it counts up to the voltage of thereference voltage source 128 of the succeeding stage comparator 127, inthe normal state, i.e., in the normal state of the commercial AC powersource 1. In the normal state therefore, an output of the comparator 127takes always a Low level. Hence, a set input S and an output Q of the RSflip flop circuit 124 are always Low. In the normal state, an output ofthe AC power failure detection circuit 12 is, therefore, always Low.

While an output of the AC power failure detection circuit 12 is Low, theswitch 10 maintains off whereas the charge/discharge control circuit 13takes a charge mode for charging the rechargeable battery 9. In thismode, the charge/discharge circuit 8 lowers the voltage of 48 V to 54 Vat the output point (c) to charge the rechargeable battery 9. Therechargeable battery 9 is preferably a serial connection of 25 to 30cells of, for example, an NiMH battery. The number of serially connectedcells is set so that the maximum value of the voltage of charged cellswill not exceed the voltage at the output point (c) of thecharge/discharge circuit and that the voltage reduction ratio becomessmall.

The following operations are performed after the commercial AC powersource 1 fails. In the AC power failure detection circuit 12 shown inFIG. 2, the voltage at the output point (a) of the full waverectification circuit becomes 0 so that the output of the hysteresiscomparator 120 takes the Low level. Then, the input of the clear CLR ofthe counter 123 is reduced to zero so that the output of the counter 123and the D/A comparator 126 exceeds a reference voltage Vref1 of thereference voltage source 128 and the comparator 127 is inverted to aHigh level. The output Q of the RS flip flop circuit 124, i.e., theoutput of the AC power failure detection circuit 12, takes therefore theHigh level. In this manner, the AC power failure detection circuit 12detects a power failure.

Upon the detection of a power failure, in the circuit shown in FIG. 1,the switch 10 turns on, which otherwise normally off. The control of thecharge/discharge circuit 8 by the charge/discharge control circuit 13 isswitched from the charge mode to the discharge mode.

In this discharge mode, the charge/discharge circuit 8 raises thevoltage of the rechargeable battery 9 and apply it to the output point(c) of the charge/discharge circuit. A voltage command value is set tothe charge/discharge circuit 8 in such a manner that the voltage at theoutput point (c) becomes higher than that in the normal power state. Forexample, the voltage at the output point (c) of the charge/dischargecircuit is set to DC 100 V in the discharge mode. This voltage is setlower than the reference voltage VH of the reference voltage source 121of the hysteresis comparator 120.

This DC voltage 100 V at the output point (c) of the charge/dischargecircuit is applied via the switch 10 and diode 11 to the output point(a) of the full wave rectification circuit, and raised to DC 380 V bythe power factor correction converter 6. The power factor correctionconverter 6 is controlled so that it has a current command valueproportional to the amplitude of voltage at the output point (a) of thefull wave rectification circuit. The power factor correction converter 6operates therefore at a constant current command value after the voltageat the output point (a) of the full wave rectification circuit (a)becomes constant.

Upon reception of stable DC 380 V, the multi-output DC/DC converter 7continues to operate stably and supplies a power to the load 3 withoutinterruption.

In this discharge mode, the voltage at the output point (c) of thecharge/discharge circuit 8 is controlled to be at DC 100 V by thecharge/discharge circuit 8. As a result, of the outputs from themulti-output DC/DC converter 7, the output to the output point (c) ofthe charge/discharge circuit is not supplied because a reverse bias isapplied to the diode 77.

As described above, according to this embodiment, the voltage at thecharge/discharge circuit output point (c) is raised in the power failurestate to prevent a circulation from the charge/discharge circuit outputpoint (c)→power factor correction converter 6→multi-output DC/DCconverter 7→charge/discharge circuit output point (c).

When the power failure is recovered, this recovery is judged because avoltage higher than DC 100 V is applied to the hysteresis converter 120in the AC power failure detection circuit 12 shown in FIG. 2. Thecounter 123 is thus reset. At the same time, a pulse output from thefilter 122 is applied to the reset input terminal R of the RS flip flopcircuit 124 to invert the output of the AC power failure detectioncircuit 12 to the Low level to thus detect the recovery from the powerfailure. After the power failure is detected, the switch 10 is turnedoff and the charge/discharge circuit 8 is switched to the charge mode torecover the normal operation.

FIGS. 4A and 4B are diagrams explaining a flow of electric energyaccording to the embodiment of the invention. FIG. 4A shows the energyflow while the AC power source 1 is normal. A power is supplied to theload 3 from the commercial AC power source 1 via the full waverectification circuit 5, power factor correction converter 6 andmulti-output DC/DC converter 7. The rechargeable battery 9 is charged bythe multi-output DC/DC converter 7 via the charge/discharge circuit 8.

FIG. 4B shows the flow while the AC power source 1 fails. A power issupplied to the load 3 from the rechargeable battery 9 via thecharge-discharge circuit 8, power factor correction converter 6 andmulti-output DC/DC converter 7.

The main structure of the embodiment will be summarized in thefollowing. The main circuitry includes the rectification circuit 5connected to the AC power source 1, the boost type converter 6 forreceiving an output of the rectification circuit 5 to suppress inputcurrent harmonics under switching control, and the output converter 7for converting an output of the boost type converter 6 into desired DCvoltages and supplying a power to the load 3. Power failure is dealtwith the uninterruptible power system 2 having the structure constitutedof the energy accumulation unit (rechargeable battery) 9 and the boostunit (charge/discharge circuit or boost chopper) 8 for feeding a powerfrom the energy accumulation unit to the output converter 7. The boostunit 8 boosts a power supplied from the energy accumulation unit 9 andsupplies a power to the input side (a) of the boost type converter 6.

One of the outputs of the output converter 7 is applied to the highvoltage side (c) of the boost unit 8, and a bi-directional DC/DCconverter capable of lowering a voltage in a reverse direction is usedas the boost unit 8. While the AC power source 1 is normal, thisbi-directional DC/DC converter 8 charges the energy accumulation unit 9by activating its voltage lowering operation. While the AC power source1 fails, a power from the energy accumulation unit 9 is boosted andsupplied to the input side (a) of the boost type converter 6.

The switch 10 is provided between the input side (a) of the boost typeconverter 6 and the bi-directional DC/DC converter 8. While the AC powersource 1 is normal, the switch 10 is turned off to charge the energyaccumulation unit 9 by the voltage lowering operation of thebi-directional DC/DC converter 8. While the AC power source 1 fails, theswitch 10 is turned on to supply the energy in the energy accumulationunit 9 to the boost type converter 6 via the switch 10 by the voltageraising operation of the bi-directional DC/DC converter 8.

In this embodiment, although an NiMH battery is used as the rechargeablebattery 9, other rechargeable batteries such as a Li-ion battery and alead battery, or an electric double layer capacitor may also be used. Asthe AC power failure detection circuit 12, other circuits forsynchronizing with the voltage phase of the commercial AC power source1, such as a phase locked loop (PLL), may be used. The switch 10 may bea semiconductor switch such as a power MOSFET and an IGBT. Otherswitches such as a relay may also be used. In place of the diode 11 andswitch 10, a semiconductor switching device having a reverse breakdownvoltage may be used.

FIG. 5 is a circuit diagram showing the specific structure of theuninterruptible power system according to the embodiment of theinvention. The commercial AC power source 1 is connected to the noisefilter 4 in UPS 2. The noise filter 4 is constituted of a common-modechoke 41 and capacitors 42 to 45. An output of the noise filter 4 issupplied to the full wave rectification circuit 5. The full waverectification circuit 5 is a bridge circuit made of diodes 51 to 54. Anoutput point on the high potential side of the full wave rectificationcircuit 5 corresponds to the output point (a) of the full waverectification circuit, the output point being connected to the powerfactor correction converter 6 and AC power failure detection circuit 12.The power factor correction converter 6 is a boost converter constitutedof a choke coil 61, a power MOSFET 62, a diode 63 and an outputcapacitor 64. The output point (c) of the power factor correctionconverter is connected to the multi-output DC/DC converter 7. Thisconverter 7 has a power MOSFET 72 on the primary winding side of amulti-coil transformer 71 and rectification and smooth circuits 73 to 76on the secondary winding side. The rectification and smooth circuit 73to 75 are connected to the load 3 to supply powers having different DCvoltages. The rectification and smooth circuit 76 has the diode 77 shownin FIG. 1 and is connected to the output point (c) of thecharge/discharge circuit 8.

The charge/discharge circuit 8 is a DC/DC converter called a binantconverter or bi-directional converter, and is a voltage raising/loweringchopper constituted of a choke coil 81, power MOSFETs 82 and 83 and acapacitor 84.

The rechargeable battery 9 is connected to the lower voltage side of thecharge/discharge circuit 8. Connected to the output point (c) of thecharge/discharge circuit 8 is the source of a p-channel power MOSFET asthe switch 10. The drain thereof is connected to the anode of the diode11. The cathode of the diode 11 is connected to the output point (a) ofthe full wave rectification circuit.

FIG. 6 is a diagram showing the structure of an uninterruptible powersystem according to a second embodiment of the invention. In FIG. 6,like constituent elements to those shown in the other drawings arerepresented by identical reference numerals and the duplicatedescription thereof is omitted. Different points from those shown inFIG. 1 reside in that a fuel battery 901 is used as the energyaccumulation unit, that a discharge circuit (boost converter) 801 isused in place of the charge/discharge circuit 8 and that a chargecontrol circuit 131 is used in place of the charge/discharge controlcircuit 13. Because of this structure, a charge output from themulti-output DC/DC converter 7 does not exist and the switch 10 is notneeded. While the commercial AC power source 1 is normal, an output ofthe AC power failure detection circuit 12 is Low and the boost converter801 is in a stop state. Since the fuel battery 901 is not necessary tobe charged if fuel is properly supplied, the charge control is notnecessary in the normal power source state, which is different from thefirst embodiment. The operation during the power failure is similar tothat of the first embodiment.

In the second embodiment, the structure of the multi-output DC/DCconverter 7 can be simplified. The charge/discharge circuit 8 shown inFIG. 1, i.e., the bi-directional DC/DC converter and the charge controlcircuit, are not necessary.

In the second embodiment, in addition to the fuel battery, other varioustypes of batteries may also be used including a primary battery, a solarbattery and the like.

According to the above-described embodiments of the invention, theembodiments can have the merit that the number of serially connectedbattery cells can be made smaller than a conventional multi-outputconverter having a backup function. The volume of UPS can therefore bereduced and a low cost can be realized. As the battery charge/dischargecircuit, a non-isolated type bi-directional DC/DC converter with asimple structure can be used so that the circuit can be made compact andinexpensive. The assembly densities of a system, an informationprocessing apparatus and a server can be improved.

Since a lead battery is not used but a nickel metal hydride battery canbe used, a safe system can be provided by mitigating an environment loadto be incurred by dumping lead batteries.

According to the invention, an uninterruptible power system (UPS) can beprovided which can lower the voltage of the energy accumulation unit.The number of serially connected battery cells can be made smaller thana conventional multi-output converter having a backup function. Thevolume of UPS can therefore be reduced and a low cost can be realized.Since the number of serially connected cells can be reduced, thereliability against failure of battery cells can be improved.

As the battery charge/discharge circuit, a non-isolated typebi-directional DC/DC converter with a simple structure can be used sothat the circuit can be made compact and inexpensive. The assemblydensities of a system, an information processing apparatus and a servercan be improved.

Since a lead battery is not used but a nickel metal hydride battery canbe used, a safe system can be provided by mitigating an environment loadto be incurred by dumping lead batteries.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An uninterruptible power system for inputting an AC power, convertingthe AC power into a desired power, supplying the desired power to aload, and upon occurrence of an AC failure, converting a DC power fromenergy accumulation means into the desired power and supplying thedesired power to the load, wherein the DC power from said energyaccumulation means is passed through two serially connected boost means,and thereafter the power is converted into the desired power to besupplied to the load, wherein first boost means of the two seriallyconnected boost means, which is nearer to the energy accumulation means,raises the DC electric power from said energy accumulation means tosupply second boost means of the two serially connected boost means whensupply of electric power is interrupted, and lowers inputted electricpower for charging the electric accumulation means during an ordinaryoperation.
 2. An uninterruptible power system according to claim 1,wherein first boost means is a boost converter for receiving an outputof said energy accumulation means, boosting the output by switchingcontrol and supplying the boost output to the second boost means.
 3. Anuninterruptible power system according to claim 1, wherein one of saidtwo boost means is a boost converter for receiving an output of arectification circuit and suppressing input current harmonics of said ACpower by switching control.
 4. An uninterruptible power system accordingto claim 3, wherein the other of said two boost means is a boostconverter for receiving an output of said energy accumulation means,boosting the output by switching control, and supplying the boost outputto an input side of said boost converter.
 5. An uninterruptible powersystem according to claim 1, comprising: a rectification circuitconnected to an AC power source; the first boost means for boosting anoutput of said rectification circuit; a multi-output converter forconverting an output of said first boost means into a plurality ofdifference DC voltages; and the second boost means for feeding a powerin said energy accumulation means to said first boost means uponoccurrence of an AC power failure.
 6. An uninterruptible power systemaccording to claim 5, wherein said first boost means is a boostconverter for receiving an output of said rectification circuit andsuppressing input current harmonics of said AC power by switchingcontrol.
 7. An uninterruptible power system according to claim 5,wherein said second boost means is a boost converter for receiving anoutput of said energy accumulation means, boosting the output byswitching control, and supplying the boost output to an input side ofsaid boost converter.
 8. An uninterruptible power system comprising: arectification circuit connected to an AC power source; a boost converterfor receiving an output of said rectification circuit and suppressinginput current harmonics by switching control; an output converter forconverting an output of said boost converter into a desired DC voltageto be supplied to a load; energy accumulation means; and boost means forfeeding a DC power in said energy accumulation means towards said outputconverter upon occurrence of an AC power failure, wherein said boostmeans boosts the DC power in said energy accumulation means and feedingthe boost power to an input side of said boost converter to supply saidboost converter when supply of electric power is interrupted, and lowersinputted electric power for charging the electric accumulation meansduring an ordinary operation.
 9. An uninterruptible power systemaccording to claim 8, wherein said energy accumulation means is arechargeable battery, an electric double layer capacitor or a fuel cell.10. An uninterruptible power system according to claim 8, wherein saidoutput converter converts an output of said boost converter into aplurality of different DC voltages and supplying the voltages to theload.
 11. An uninterruptible power system according to claim 10, whereinsaid energy accumulation means is a rechargeable battery, an electricdouble layer capacitor or a fuel cell.
 12. An uninterruptible powersystem comprising: a rectification circuit connected to an AC powersource; a boost converter for receiving an output of said rectificationcircuit and suppressing input current harmonics of said AC power byswitching control; an output converter for converting an output of saidboost converter into a desired DC voltage to be supplied to a load;energy accumulation means; and boost means for feeding a DC power insaid energy accumulation means towards said output converter uponoccurrence of an AC power failure, wherein a portion of an output sideof said output converter is coupled to a high voltage side of said boostmeans, said boost means is a bi-directional DC/DC converter capable of aback mode operation in a reverse direction, said boost means lowersinputted electric power for charging said energy accumulation meanswhile the AC power source is normal, and while the AC power sourcefails, said boost means boosts the DC power in said energy accumulationmeans and supplies the boost power to an input side of said boostconverter.
 13. An uninterruptible power system according to claim 12,wherein a voltage on a high voltage side of the bi-directional DC/DCconverter is set higher while the AC power source fails than while theAC power source is normal.
 14. An uninterruptible power system accordingto claim 12, wherein switch means is provided between an input side ofsaid boost converter and a high voltage side of said bi-directionalDC/DC converter, while the AC power source is normal, said switch meansis turned off and said bi-directional DC/DC converter performs a voltagelowering operation to charge said energy accumulation means, and whilethe AC power source fails, said switch means is turned on and saidbi-directional DC/DC converter performs a voltage raising operation tosupply an energy in said energy accumulation means to said boostconverter via said switch means.
 15. An uninterruptible power systemaccording to claim 14, wherein a voltage on a high voltage side of thebi-directional DC/DC converter is set higher while the AC power sourcefails than while the AC power source is normal.
 16. An uninterruptiblepower system according to claim 12, wherein said output converterconverts an output of said boost converter into a plurality of differentDC voltages to be supplied to the load.
 17. An uninterruptible powersystem according to claim 16, wherein switch means is provided betweenan input side of said boost converter and a high voltage side of saidbi-directional DC/DC converter, while the AC power source is normal,said switch means is turned off and said bi-directional DC/DC converterperforms a voltage lowering operation to charge said energy accumulationmeans, and while the AC power source fails, said switch means is turnedon and said bi-directional DC/DC converter performs a voltage raisingoperation to supply an energy in said energy accumulation means to saidboost converter via said switch means.
 18. An uninterruptible powersystem according to claim 17, wherein a voltage on a high voltage sideof the bi-directional DC/DC converter is set higher while the AC powersource fails than while the AC power source is normal.